νόμον οὖν καταργοῦμεν διὰ τῆς πίστεως; … νόμον ἱστάνομεν

Says 3:31: νόμον οὖν καταργοῦμεν διὰ τῆς πίστεως; μὴ γένοιτο, ἀλλὰ νόμον ἱστάνομεν (BSB "Do we, then, nullify the law by this faith? Certainly not! Instead, we uphold the law"). καταργέω is "to render inoperative or invalid" (Abbott-Smith, Rom 3:3, 31), the verb used of God's faithfulness in 3:3. ἱστάνω (histanō) is a late by-form of ἵστημι (Abbott-Smith), here "to establish, uphold"; the Byzantine text has ἱστῶμεν (var.Rom.3.31.histanomen).

Says διὰ τῆς πίστεως has the anaphoric article: "this faith" just described (3:22–30).

Uncertain How faith upholds the law. This note leans to the law as Scripture bearing witness to justification by faith (3:21, "attested by the Law and the Prophets"; chapter 4 follows with Abraham). The main alternative is that faith leads to the fulfillment of the law's requirement (8:4). The two can be held together.